The rare earth based permanent magnets, especially R-T-B based sintered magnets, are widely used in various electric equipments because of exhibiting excellent magnetic properties. However, several technical problems to be solved exist in the R-T-B based sintered magnets with excellent magnetic properties. One of the problems is that coercivity significantly decreases accompanied with increase in temperature due to low thermal stability. Therefore, the coercivity at room temperature can be elevated by the addition of heavy rare earth elements with Dy, Tb and Ho as the representative. Thus, as disclosed in Patent Document 1 (JP5-10806), even if the coercivity decreases as temperature rises, it will be enough for use. Compared to the R2T14B compound using light rare earth elements such as Nd, Pr and the like, the R2T14B compound with the addition of these heavy rare earth elements has a high magnetic anisotropy field and can obtain a high coercivity.
The R-T-B based sintered magnet consists of the main phase crystal grains and the sintered body, wherein the main phase crystal grain is composed of the R2T14B compound, and the sintered body at least comprises a grain boundary phase containing more amount of R than the main phase. In Patent Document 2 (JP7-122413) and Patent Document 3 (WO2006/098204), the optimal concentration distribution of the heavy rare earth elements in the main phase crystal grains which greatly affects the magnetic properties has been disclosed as well as the control method thereof.
It is said in Patent Document 2 that in the rare earth based permanent magnet with the main phase, which has the R2T14B compound (R represents one or two or more rare earth elements, and T represents one or two or more transition metals) as the main body, and the R-rich phases (R represents one or two or more rare earth elements) as the main constituent phases, the heavy rare earth elements are distributed at a high concentration in at least three sites of the main phase grains. The R-T-B based sintered magnet disclosed in Patent Document 2 was obtained by respectively pulverizing a R-T-B based alloy with the R2T14B compound as the main constituent phase and a R-T based alloy with a area ratio of R-T eutectic crystal being 50% or less which contains at least one kind of heavy rare earth elements, then mixing, molding and sintering the molded body. This R-T-B based alloy preferably has the R2T14B compound as the main constituent phase, and such a composition is recommended as 27 wt % (mass %)≦R≦30 wt % (mass %), 1.0 wt % (mass %)≦B≦1.2 wt % (mass %) and T of the balance.
Patent Document 3 has disclosed that an R-T-B based sintered magnet can be obtained with both a high residual flux density and a high coercivity if the following conditions are satisfied. That is, the crystal grain contains the R2T14B compound as the main body and comprises at least one of Dy and Tb, which are heavy rare earth elements, and at least one of Nd and Pr, which are light rare earth elements; the crystal grain also has a core-shell structure comprising a inner shell portion and a outer shell portion that covers the inner shell portion; in the crystal grain, the concentration of the heavy rare earth elements in the inner shell portion is lower than that in the periphery of the outer shell portion by 10% or more; when the shortest distance between the periphery of the crystal grain and the inner shell portion is set as L and the equivalent circle diameter of the crystal grain is set as r, the average of L/r ranges from 0.03 to 0.40; at the cross-section of the crystal grain, the number of the crystal grains with the core-shell structure accounts for 20% or more based on the number of total crystal grains forming the sintered body.